Heat sink



FIG. 1 is a perspective view of a heat sink of a lighting system showing our new design;

FIG. 2 is a first side elevational view thereof;

FIG. 3 is a second side elevational view thereof;

FIG. 4 is a third side elevational view thereof;

FIG. 5 is a fourth side elevational view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom plan view thereof.

The broken lines shown in the drawings are included for the purpose of illustrating elements of the heat sink of a lighting system and form no part of the claimed design. 

CLAIM The ornamental design for a heat sink, as shown and described. 